Known methods for preparing chloroformyl-substituted benzenes mainly include the photochlorination method (see DE3146868 and JP57-130931), sulfoxide chloride method, phosphorus trichloride method, phosphorus pentachloride method, and phosgene method. The sulfoxide chloride method is the most commonly used (see, for example, CN102516060A and CN102344362A). However, the method requires the use of phthalic acid having a high purity of 99.99% as a raw material, which results in high cost. Moreover, these methods all suffer from a problem of generation of environmentally unfriendly by-products such as hydrogen chloride, sulfur dioxide, carbon dioxide, and phosphorous acid. These by-products cause inconvenience to subsequent treatments of products and likely lead to environmental pollution.
The photochlorination method may use methyl aromatic compounds as raw materials, but the amount of the by-product hydrogen chloride is enormous. How to deal with the large amount of hydrogen chloride has become an issue to be urgently solved. Currently, the main measure actually adopted in the industry is to absorb hydrogen chloride with water to prepare low-value and inexpensive hydrochloric acid for sale; since hydrochloric acid is inexpensive and has limited market demand, the preparation of hydrochloric acid from hydrogen chloride has become a burden rather than a resource. Another measure is to neutralize hydrogen chloride with a base for direct discharge; however, with increasing sophistication of environmental laws and regulations, environmental protection standards of various ways of discharge have become very stringent.
The method for preparing chlorine gas directly from the by-product hydrogen chloride can not only achieve closed circulation of chlorine element, but also achieve zero emissions in the reaction process, which greatly improves the level of energy saving and emission reduction in the industry, reduces the cost, and eliminates pollution to the environment. Up to now, the methods of preparing chlorine gas from hydrogen chloride can be divided into three main categories: electrolytic method, direct oxidation method, and catalytic oxidation method. The electrolytic process has a high energy consumption and uses an ionic membrane that needs to be frequently replaced, resulting in a very high cost, wherein the cost per ton chlorine gas recovered is greater than 4,000 RMB Yuan. The direct oxidation method suffers from a low yield and cannot be industrialized. In contrast to the electrolytic method and the direct oxidation method, the catalytic oxidation method, particularly the Deacon catalytic oxidation, exhibits the highest potential for industrialization.
Chlorine gas used in many productions in the chlorination industry is required to be ≥99.6% (vol %). Therefore, for chlorine gas obtained from the Deacon reaction, a high-purity chlorine gas capable of being recycled can be obtained only after a problem of separation of the resulting mixed gas from the reaction is solved. Particularly, to recover chlorine gas from a mixed gas from oxidation of hydrogen chloride, a separation method of hydrogen chloride by absorption with water is generally adopted, for examples, CN102502498A and US2008/0159948A1, which generates a large amount of dilute hydrochloric acid and thus require further treatments.
In view of the above, there is a need in the art for a clean process for producing chloroformyl-substituted benzenes with low cost, good quality, and no pollution that is capable of achieving closed circulation of chlorine resources. The key of the clean process lies in the oxidation process of hydrogen chloride and separation process of the product gas stream. Once the critical processes are solved, a high-purity chlorine gas may be obtained, thus achieving recycle of chlorine gas. The clean production process of chloroformyl-substituted benzenes is the key to achieve industrialization of related chemical industries, for example, the aramid industry.